A MEETING WITH THE UNIVERSE

Chapter 2-3

The Gas Giants

Great storms of Jupiter.
An exercise in cosmic modern art, huge whirling storms and sawtoothed,
turbulent flows spread out in Jupiter's atmosphere as pictured by Voyager
2 from 6 million kilometers (3.7 million miles). The Red Spot (right center)
is a huge storm system, big enough to hold three Earths, that has persisted
for at least three centuries. It whirls counterclockwise, producing highly
contorted patterns at its left, where cloud banks moving left to right
are blocked and forced to squeeze past it. Smaller white oval storms,
about the size of Earth, create similar turbulent effects below the Red
Spot. Most patterns in Jupiter's atmosphere are constantly changing; the
structures shown here have changed significantly since Voyager 1 photographed
them four months previously. Jupiter's atmosphere is composed almost entirely
of colorless hydrogen and helium; the colors come from small amounts of
unknown substances, perhaps compounds of sulfur and phosphorus.

Jupiter

Jupiter is where the action is. The
planet is big enough to hold 1400
Earths and is almost 2.5 times more
massive than all of the other planets
put together. It is a huge, rapidly spinning
blob of cold gases - hydrogen,
helium, a little methane, water, and
ammonia - all colored by traces of
more complex but largely unknown
chemicals. The outer part of the
atmosphere is all that we see of Jupiter.
There, clouds form in belts and
stripes, in consequence of the fast
rotation. This planet, 11 times the
diameter of Earth, spins on its axis in
less than half an Earth day.

Deep inside Jupiter's atmosphere,
the pressure becomes enormous, and
hydrogen, a gas at the higher altitudes,
is condensed to a liquid. At
about 25,000 kilometers (16,000
miles) below the cloud tops, the pressure
attains a value 3 million times
that of the Earth's atmosphere at sea
level. At this pressure, the liquid hydrogen
transforms to a metallic state.

Currents circulating through the me
tallic hydrogen fluid generate a mag
netic field that is about 14 times
stronger at Jupiter's cloud tops than
the field at the surface of the Earth.
Disturbances in the magnetic field
around Jupiter produce powerful
bursts of radio waves, making Jupiter
the noisiest radio transmitter in the
solar system, other than the Sun.
Jupiter's magnetic field is much
stronger than the Earth's, and it is
much less compressed by the solar
wind, the stream of charged atoms
that pour out from the Sun. Jupiter's
field forms a huge magnetosphere
around the planet, and the fleld
sweeps out on the side opposite the
Sun to give Jupiter a fat "magnetic
tail" at least 150 times wider than the
diameter of the planet itself.

A menagerie of moons.
Jupiter's four largest moons,first seen as tiny dots of light in Galileo's
telescope, are revealed as strange new worlds by the cameras of Voyagers
1 and 2. Seen here in their relative proportions, they show a bewildering
variety. Each is different from our own Moon and different from the others.
Io is a red-orange world, pitted by the craters of active volcanoes that
constantly renew its surface with sulfur and sodium compounds.

Europa is a yellowish,
smooth globe, crisscrossed with dark lines
that may be the fractures
of an icy crust. Ganymede, larger than our Moon,
has light and dark
regions dotted by bright impact craters
that may have exposed a
subsurface ice layer. Callisto,
brownish and heavily cratered, has
perhaps the oldest planetary surface
yet discovered, its landscape
sculptured by an intense meteorite bombardment during the
formative stages of the solar system.

Jupiter's composition of hydrogen
and helium is the same as the
Sun's. Recent theoretical studies have
carried the similarity even further,
suggesting that if Jupiter had been
only a little larger, it might have be
come another Sun, the gas at its center
compressed so much that nuclear
reactions would have begun. Thus,
Jupiter is a sort of "star that failed",
although it still radiates, by compressing
its own core, twice the amount of
energy that it receives from the Sun.

The most striking resemblance, however,
is that Jupiter is the center of a
miniature solar system, surrounded
by an array of at least 16 moons, just
as the Sun is surrounded by planets,
comets, and a belt of asteroids.

Jupiter, remarkable even as observed
from Earth, revealed even more
wonders as our spacecraft sped past it
in recent years. Cameras gave us a
close view of its banded atmosphere,
and we learned that the seemingly
smooth belts of color were actually
separated by zones of violent turbulence.
Earth-sized blobs of colored gas
travel along huge "jet streams", spin,
and collide. Great Plumes rise from
deep in the atmosphere and leave
trails 10,000 kilometers (over 6200
miles) long across the planet. The
great Red Spot, a long-lived oval as
big as three Earths (it was first seen
at least 300 years ago), is a complex,
swirling hurricane. Even the night
side of Jupiter is alive with the flash
of lightning superbolts and the glowing bands of huge aurorae.

The spacecraft passing Jupiter
discovered three new moons, adding
to the 13 already observed from Earth.
Inside the orbit of the innermost
moon they found a ring, a thinner version
of the rings of Saturn, never
seen from Earth. The ring, made of
tiny particles, apparently reaches
right down to the cloud tops of Jupiter,
and it has stimulated a major debate
about how it formed and how it
continues to exist so close to Jupiter.

The moons of Jupiter are equally
spectacular, remarkably different
from our own Moon and from each
other. The two Voyager spacecraft
discovered that they are very unlike
what astronomers expected. Amalthea,
once considered Jupiter's innermost
moon, is an irregular rock about
270 kilometers (170 miles) long, but
only about 155 kilometers (96 miles)
wide along one axis. Beyond it are the
four Galilean satellites (so-called
because Galileo discovered them).

Three of them are larger than our own
Moon, including Ganymede, which is
even bigger than the planet Mercury.

Volcanoes of Io.
Jupiter's moon Io displays the only active volcanoes
found outside the Earth. Driven by tidal heating
as Io circles mighty
Jupiter, the volcanic eruptions are
still shaping the moon's surface.
They spray sodium and sulfur atoms, making
a cloud that surrounds Io's orbit. In this
computer-enhanced picture from Voyager 1,
blue plume on the horizon consists
of material hurled upward from
volcano to more than 150 kilometers
(about 90 miles) above Io's
blotchy red-orange landscape.

Io, the innermost Galilean satellite,
was one of the biggest surprises
of the Space Age, a brilliant red
orange world with more than 100 volcanoes.

The two Voyagers saw seven
of the volcanoes in continual eruption,
shooting umbrella-shaped plumes of
sulfur particles high above Io's sur
face. These unique volcanoes, the
only active ones known outside the
Earth, seem to be ejecting molten sulfur
at a temperature of a few hundred
degrees, not molten rock at more than
1000° C. The heat comes, not from
radioactivity as in the Earth, but from
tidal forces and perhaps also huge
electric currents that act on Io as it
swings around Jupiter.

Europa, the next moon out, resembles
a smooth, yellowish billiard
ball and comprises perhaps the flat
test real estate in the solar system. It
is crisscrossed with thin lines, some
several thousand kilometers long, that
remind some observers of a kind of
modern art. The lines may be the
traces of a shifting icy crust, but we
have no idea of what forces have
acted, and continue to act, to keep
this world so flat.

Ganymede, the largest of Jupiter's
moons, looks a little more familiar.
Like our own Moon, its surface is
divided into light and dark regions,
and we can see craters. There the
similarity ends. Ganymede is a light
weight moon, a "snowball" of mixed
ice and rock. Bright rays extend from
the craters, perhaps consisting of fresh
ice. The light areas of Ganymede display
long bands of parallel grooves,
unique and puzzling structures which
indicate that internal forces have
shaped the surface.

Callisto, the outermost Galilean
moon, is a heavily cratered, brownish
world. It seems to be a snowball
moon like Ganymede, but its surface
is uniformly cratered and preserves a
long history without any disruption.

Callisto may have the oldest surface
yet observed in the solar system, its
craters perhaps dating back to an ancient
era of bombardment, which
ended about 4 billion years ago. Callisto
also bears what may be the largest
crater anywhere in the solar
system, a huge, multi-ringed basin
more than 2500 kilometers (1600
miles) across.

Even the space around Jupiter
turns out to be exciting. Jupiter's huge
magnetic field is in a constant struggle
with the streams of charged particles
that speed outward from the Sun.
As a result, the Jovian magnetosphere
contains regions of highly charged
trapped radiation particles, like the
Earth's Van Allen belts, but much
larger. The four large moons lie with
in this belt of radiation, and it affects
them. Sulfur and sodium atoms
blasted out of Io by volcanoes form a
glowing, doughnut-shaped band in
Io's orbit around Jupiter. An intense
3-million ampere electric current links
Io to the top of Jupiter's atmosphere,
continuously flowing from moon to
planet and back again. Further out,
beyond the moons, there is a region of
space where the atomic particles are
so energetic that their temperature
has risen to about 3 million degrees,
the hottest place in the solar system
except for the Sun.

The Jovian system of Jupiter
and its moons is a place where all the
forces of the solar system - atmospheres,
volcanoes, cratering, magnetism,
charged particles, radiation - are
present on scales so vast that they
inspire excitement and awe. We have
seen only partially and briefly the
wonders that exist there, and we are
only beginning to understand it all.